How to create a `files`-parameter for the bpy.ops.sequencer.image_strip_add()-operator?
Don't use operators to do this; they aren't intended to be called from Python.

The regular data/function API is much simpler to use:

import bpy

se = bpy.context.scene.sequence_editor_create()
seq = se.sequences.new_image('hey', 'C:/temp/hey.png', 1, 1)